This invention relates to a system including a variable rate modulator for (1) varying the rate at which signals are modulated in accordance with variations in the rate of introduction of digital data to the system and (2) for processing the modulated signals to provide output signals at a fixed sampling frequency.
In recent years, a number of different applications have arisen in which digital signals representing data are processed and the processed signals are then converted to analog signals. For example, such applications have included the transmission of television signals through coaxial lines to homes. In such systems, the digital data is converted to analog signals and the analog signals are then transmitted through coaxial lines to homes of subscribers. Other applications are in microwave links and satellite communications.
In a number of the different applications involving the processing of digital data and the conversion of the processed digital data to analog signals, the digital data is provided at a variable input frequency or rate and the analog signals are provided at a fixed sampling frequency different from the variable input frequency or rate. For example, the digital data may be provided in the range of approximately 0.1-20 megabits per second and the analog signals may be sampled at a fixed frequency in the range of approximately 100-200 megahertz.
In the above example, the variable rate digital signals in the range of 0.1-20 megabits/second are converted to a modulated analog intermediate frequency signals having a fixed sampling frequency. For example, the digital signals in the range of 0.1-20 megabits/second may be converted to signals at a fixed sampling frequency of approximately 100-200 megahertz. The signals at the sampling frequency are then modulated onto a programmable carrier frequency in the range of approximately 5-65 MHz. at the fixed sampling frequency of approximately 100-200 megahertz.
As will be seen from the above discussion, a considerable range of frequencies (e.g. 0.1-20 megabits/second) have to be converted to a single fixed frequency (e.g. 120 megahertz). This is not easy. If the conversion is not accurate, the signals at the fixed sampling frequency jitter. When the signals illustratively provide television information, the jitter produces a significant deterioration in the quality of the television image.
This invention provides a system for, and method of, converting digital data signals variable through a wide range of frequencies or rates into signals at a fixed sampling frequency. This conversion occurs without any jitter in the signals at the fixed sampling frequency. When the system of this invention is illustratively used to provide television images, the television images have a high resolution.
In one embodiment of the invention, digital data signals at a variable input frequency are converted by a numerically controlled oscillator and an interpolator to a signal at a fixed sampling frequency. The conversion of the variable input frequency to the fixed output sampling frequency may be by a factor other than an integer.
The interpolated digital data signals at the output sampling frequency are then modulated onto a pair of trigonometric signals at a programmable carrier frequency, one signal having a cosine function and the other signal having a sine function.
The modulated pair of trigonometrically related signals at the fixed sampling frequency are then combined to create a modulated signal at a carrier frequency determined by the frequency of the sine and cosine signals. The modulated signal is sampled at the fixed sampling frequency and converted to a corresponding analog signal using a digital-to-analog converter.